1. Field of the Invention
The present invention relates to measurement of fine particles (foreign matter) present in a process unit for formation of films, etching, cleaning, etc.
2. Description of the Prior Art
FIG. 7 shows the arrangement of a conventional fine-particle measuring apparatus disclosed, for example, in Japanese Patent Post-Exam Publication Number 63-30570 (1988). The apparatus is designed to measure fine particles attached to the surface of a wafer. In the figure, reference numeral 1 denotes a substrate for a semiconductor device, that is, a wafer, which is an object of measurement, 2 a fine particle, 3 a laser light source (a light source for generating parallel rays), 4 a polarizer, 5 objective lenses, 6 a photodetector which converts light into an electric signal, and 7 an electronic circuit device which processes information output from the photodetector 6 to obtain the results of measurement of fine particles. The reference numeral 8 denotes a driving mechanism for moving the position of the wafer 1.
The operation of the prior art will next be explained. Laser light emitted from the laser light source 3 is applied parallel to the surface of the wafer 1. As the laser light that is applied to the wafer surface, S-polarized laser light is employed. The S-polarized laser light is scattered by the fine particle 2. However, since the surface of the fine particle 2 has minute irregularities, the scattered light contains a large amount of P-polarized light component. On the other hand, the medium that constitutes the measuring atmosphere is usually a gas,m i.e., air, and light that is scattered by gas molecules in the manner of Rayleigh scattering contains no P-polarized light component. Accordingly, the scattered light caused by gas molecules is cut off by the polarizer 4 disposed in such a manner as to intercept the S-polarized light component. As a result, only the P-polarized light component of the scattered light from the fine particle 2 is received by the photodetector 6, and the result of measurement is obtained in the electronic circuit device 7. The driving mechanism 8 is provided to measure the distribution of fine particles on the wafer surface.
The above-described prior art suffers, however, from the following problems. The fine particle under measurement should not have on its surface minute irregularities which are regarded as much smaller than the wavelength of the laser light, and it is difficult with the prior art to measure fine particles which have relatively smooth surfaces and fine particles which have relatively small particle diameters. These fine particles may be measured by the use of P-polarized laser light or non-polarized laser light in place of S-polarized laser light. In such a case, however, the Rayleigh-scattered light (P-polarized light) scattered by the gas that constitutes the measuring atmosphere cannot be cut off with the polarizer 4, and the S/N ratio cannot therefore be increased. Thus, it is difficult with the prior art to measure fine particles having small diameters. In addition, since this prior art apparatus is not adapted to measure fine particles in a process unit but is designed for off-line inspection, it is difficult to apply the prior art apparatus to measurement of fine particles in a process unit even if the polarizer 4 and the objective lenses 5 (constituting in combination a microscope) are disposed in close proximity to the wafer 1 to limit the observation zone.
FIG. 8 is a sectional view showing the arrangement of another conventional fine-particle measuring apparatus disclosed, for example, in A. Shintani et al.: J. Electrochem. Soc. 124, No. 11 (1977), pp. 1771-1776. In the figure, the reference numeral 3 denotes a laser light source, 9 a measuring zone which is spatially limited by a light-receiving lens system 10 and which contains fine particles to be measured, 7 a photodetector, and 11 an optical trap for minimizing stray light in the measuring apparatus. In use, this prior art apparatus is connected to a process unit by the use of capillary (tube) adapted to suck in a gas containing fine particles dispersed in the process unit, thereby indirectly measuring the fine particles in the process unit.
Accordingly, this prior art involves the problem that it is impossible to measure fine particles on the surface of a wafer set in the process unit and, with regard to the fine particles suspended in the process unit, it is only possible to measure those which can be successfully sucked and transported into the measuring apparatus.
FIGS. 9(a) and 9(b) are plan and front views showing the operation of PM-100 In-Situ Particle Flux Monitor, a product available from High Yield Technology. Laser light from a laser 3 is reflected a large number of times between a pair of mirrors 21 disposed parallel to each other, thereby enlarging the two-dimensional observation zone. When fine particles 2 are passing through the zone, light is scattered thereby and this scattered light is received by a photo-detector 6 to thereby measure fine particles. It should be noted that the reference numeral 22 denotes reflecting condensers, while the numeral 23 denotes a beam stopper. This apparatus is used within a process unit.
Accordingly, this prior art is capable of measuring suspended fine particles but cannot measure those attached to the wafer surface. In addition, since the optical system (comprising the laser light source 3, the mirrors 21, the photodetector 6, etc.) is installed inside a process unit, in the case, for example, of a film forming process by atmospheric pressure thermal CVD, it is difficult to measure fine particles on the surface of the wafer heated at high temperature or those which are suspended above the wafer surface during a film forming process. Even when no film is being formed, the presence of the apparatus also causes substantial changes in the environment (e.g., the gas flow, temperature distribution, etc.) in the vicinity of the wafer. In an etching or cleaning process also, it is difficult to effect measurement without causing critical disturbances. In addition, since the measuring system according to this prior art has no means for eliminating signals (i.e., background noise) caused by light that is scattered by gas molecules constituting the measuring atmosphere medium in the manner of Rayleigh scattering, it is difficult to measure fine particles having such small particle diameters that the intensity of light scattered thereby is too weak to ensure that the desired signal will not be obscured by the background noise.